1. Field of the Invention
The present invention relates generally to the measurement of the mass rate of flow for multiphase flow streams. The present invention relates more specifically to a method and apparatus for measuring the mass rate of flow for multiphase flow streams, and in particular, gas streams having entrained liquids with low (less than 15%) liquid mass fractions.
2. Description of the Related Art
Various methods have been developed and utilized for measuring the flow rate of multiphase flow streams where widely varying amounts of gas and liquid are encountered and where high degrees of measurement accuracy may or may not be important. Orifice meters are often used to meter gas flow streams by creating a pressure differential across an orifice plate from which can be calculated a mass flow rate. Various standard metering equations and commercial metering systems have been developed to meter dry gas (flow streams that are 100% gas) to accuracy levels appropriate for situations involving the sale or transfer of ownership for the material product in the flow stream. Such xe2x80x9cfiscal levelxe2x80x9d metering of dry gas provides an accuracy suitable for contract sale of gaseous products such that no significant monetary cost error occurs in the contract sale. There are clearly situations where even greater accuracy might be desired, such as in technical research and development environments, but for the purposes of the present invention, such fiscal level metering for wet gas flow streams is a suitable objective.
In many situations where flow stream metering is desired and performed, the flow stream is composed of both gases and liquids. The presence of liquids in the flow stream will result in errors in the mass flow rate measurement due to the use of equipment and measurement interpretation techniques intended primarily for dry gas flow measurements.
Production streams from gas wells and the like, flow streams in the gas leg of two or three phase separators, and flow streams in some gas transportation lines, all have entrained liquids in small to moderate amounts. These entrained liquids result from a number of factors ranging from dew point effects to moderate levels of inherent liquid fractions. From an economic standpoint, it is desirable to meter these flow streams without the necessity of conditioning the flow stream with a separator or a dehydration system. This is even more desirable in off-shore oil and gas production where the elimination of the need for a conditioning facility can significantly reduce the necessary platform space, load, and auxiliary requirements, and therefore, reduce the overall cost of the off-shore platform.
In general, by eliminating the need to condition a multiphase flow stream, several costs are avoided. These include the capital costs of the conditioning facility, the installation and maintenance cost of the conditioning facility, and the operating expenses associated with such a facility.
There have been attempts in the past to measure the mass flow rate of multiphase flow streams by a variety of more or less complex systems. Many of these systems are directed to the measurement of steam flow where condensate has an effect on the accuracy of the flow rate measurements. Many are directed to multiphase flow in the oil and gas industry. The following are representative of the state of the art in the measurement of mass flow rate for multiphase flow.
U.S. Pat. No. 1,540,533, issued to Bullock on Jun. 2, 1925, entitled xe2x80x9cFlow Meter Installationxe2x80x9d, describes an early system for utilizing pressure differential to measure flow rate with a combination orifice/venturi nozzle metering run.
U.S. Pat. No. 1,559,155, issued to Bullock on Oct. 27, 1925, entitled xe2x80x9cMultirange Flow Nozzlexe2x80x9d, describes the use of an array of flow nozzles for the creation and measurement of differential pressures.
U.S. Pat. No. 2,136,900, issued to Woolley on Nov. 15, 1938, entitled xe2x80x9cMeasuring Apparatusxe2x80x9d, describes an early flow rate system that incorporates auxiliary factors for the correction of standard parameters in measuring the flow.
U.S. Pat. No. 3,100,840, issued to Morganstern on Aug. 13, 1963, entitled xe2x80x9cMethods and Apparatus for Measuring and Testingxe2x80x9d, describes an early use of radiation in the measurement of flow rate.
U.S. Pat. No. 3,378,022, issued to Sorenson on Apr. 6, 1968, entitled xe2x80x9cFluid Flow Sensing Systemxe2x80x9d, describes a more complicated system of orifice metering and differential pressure measurement analysis.
U.S. Pat. No. 4,337,668, issued to Zupanick on Jul. 6, 1982, entitled xe2x80x9cOrifice Wear Compensationxe2x80x9d, describes a system for modifying a metering equation constant according to the deterioration of an orifice over a period of time.
U.S. Pat. No. 4,453,417, issued to Moyers et al. on Jun. 12, 1984, entitled xe2x80x9cUnitized Measurement Instrument Connector Apparatusxe2x80x9d, describes an orifice metering system specifically including an array of valves and roddable inserts to improve the operation and maintenance of the system.
U.S. Pat. No. 4,562,744, issued to Hall et al. on Jan. 7, 1986, entitled xe2x80x9cMethod and Apparatus for Measuring the Flow Rate of Compressible Fluidsxe2x80x9d, describes an orifice plate metering system that integrates temperature differentials into the standard metering equations.
U.S. Pat. No. 4,683,759, issued to Skarsvaag et al. on Aug. 4, 1987, entitled xe2x80x9cCharacterization of Two-Phase Flow in Pipesxe2x80x9d, describes the use of gamma radiation transmission measurements to determine the distribution of voids within a gas/liquid mixture flowing in a pipe.
U.S. Pat. No. 4,836,032, issued to Redus et al. on Jun. 6, 1989, entitled xe2x80x9cMethod of Determining the Quality of Steam for Stimulating Hydrocarbon Productionxe2x80x9d describes the use of an orifice plate in combination with a choke to measure both steam quality and mass flow rate.
U.S. Pat. No. 5,025,160, issued to Watt on Jun. 18, 1991, entitled xe2x80x9cMeasurement of Flow Velocity and Mass Flow Ratexe2x80x9d, describes the use of gamma radiation using dual energy transmission techniques to facilitate a more accurate measurement of flow rate.
U.S. Pat. No. 5,031,465, issued to Redus on Jul. 16, 1991, entitled xe2x80x9cSteam Quality and Mass Flow Rate Measurement Using Critical Flow Choke Upstream of an Orifice Platexe2x80x9d, also describes a pressure differential metering system that incorporates both a choke and an orifice plate.
U.S. Pat. No. 5,031,466, issued to Redus on Jul. 16, 1991, entitled xe2x80x9cMethods and Apparatus for Determining Steam Quantity and Mass Flow Ratexe2x80x9d, also describes a differential pressure measurement system utilized in conjunction with an orifice plate metering system.
U.S. Pat. No. 5,315,117, issued to Hatton et al. on May 24, 1994, entitled xe2x80x9cVolume Meter Systemxe2x80x9d, describes a system for determining the liquid or gas fraction of a two-phase flow stream.
U.S. Pat. No. 5,343,041, issued to Ruscev et al. on Aug. 30, 1994, entitled xe2x80x9cMethod and Apparatus for Determining the Physical Characteristics of a Water Flowxe2x80x9d, describes a more complex use of gamma ray radiation for measuring the flow of water along a well.
U.S. Pat. No. 5,400,657, issued to Kolpak et al. on Mar. 28, 1995, entitled xe2x80x9cMultiphase Fluid Flow Measurementxe2x80x9d, describes a multiphase metering system that incorporates densitometers in conjunction with a flow meter.
U.S. Pat. No. 5,404,745, issued Chien on Apr. 11, 1995, entitled xe2x80x9cMethod and Apparatus for Determining Steam Quality From Steam Velocity Measurementxe2x80x9d, describes a system and method for measuring the critical velocity of steam flowing through a nozzle.
U.S. Pat. No. 5,421,209, issued to Redus et al. on Jun. 6, 1995, entitled xe2x80x9cMeasurement of Steam Quality and Mass Flow Ratexe2x80x9d, describes yet another system of pressure measurement involving both an orifice plate and a critical flow venturi.
U.S. Pat. No. 5,479,020, issued to Mohn on Dec. 26, 1995, entitled xe2x80x9cMetering Device For a Fluidxe2x80x9d, describes a meter for use in multiphase flow that incorporates radiation densitometers and is directed to the more accurate use of such radiation techniques.
U.S. Pat. No. 5,501,099, issued Whorff on Mar. 26, 1996, entitled xe2x80x9cVapor Density Measurement Systemxe2x80x9d describes a metering system that attempts to return the flow stream to a homogeneous vapor state prior to metering.
At the present time, the accuracy of standard gas flow metering systems is not known well enough to allow for sound engineering decisions regarding the use of such metering systems for fiscal level metering in wet gas applications. In addition, there are currently thousands of existing well head custody transfer meter runs that are in service on wet gas streams. The accuracy of these meter runs is not known well enough to make sound determinations as to whether they function within the contract requirements associated with the custody transfer. Further, even if attempts are made to provide more accurate metering devices, the most cost effective system with which to upgrade the existing well head meter runs is not clearly known.
It would be desirable, therefore, to have a system for measuring the mass flow rate of a multiphase flow stream that includes devices and methods for correcting the most significant errors in standard metering equations that result from the presence of fluids in the multiphase flow and more particularly in the area immediately adjacent the orifice. It would be desirable for such a system to be simple to implement and require little additional intrusion on the flow stream.
It is therefore an object of the present invention to implement an apparatus and method for the measurement of the mass flow rate of a multiphase flow that compensates and corrects for the effects of the liquid component of the multiphase flow.
It is a further object of the present invention to identify and implement correction factors in standard metering equations so as to more accurately reflect upstream cross-sectional area and downstream pressure recovery to create a flow rate result that more accurately reflects the actual mass flow rate of the multiphase flow.
It is a further object of the present invention to more accurately determine the upstream cross-sectional area in a multiphase flow conduit and to utilize the more accurate cross-sectional area measurement in standard metering equations.
It is a further object of the present invention to more accurately identify the pressure recovery values downstream of an orifice meter run and to utilize these more accurate pressure values in standard metering equations.
In fulfillment of these and other objectives, the present invention overcomes the metering errors that result from the presence of liquids in standard orifice metering systems by measuring and compensating for the effects of such liquids on the flow stream. Three specific types of effects from the liquid can be readily considered as contributing to the error that results from the presence of liquids in the flow. Two separate measurements are made in the present invention and are used to correct the metered rates for the multiphase flow.
The three types of effects that are generally considered in the present invention include:
1. The reduction in upstream cross-sectional area for the multiphase flow brought about by the presence of the liquids in the flow stream conduit.
2. The change in the pressure drop across the orifice plate as a result of the transport of the liquid fraction of the multiphase flow through the orifice.
3. The change in the pressure recovery downstream of the orifice.
The two measurements that are used in the present invention to correct the metered rates when liquids are present in the flow include an upstream cross-sectional area measurement and a measurement of the pressure recovery change downstream. These measurements are used with standard orifice metering measurements to correct the flow measurement. First, the upstream gas flow cross-sectional area change is used to increase the effective coefficient Cd of the orifice run. Second, the reduced recovery of kinetic energy downstream of the orifice is used to reduce the effective coefficient Cd of the orifice meter.